A conventional linear motion bearing shown in U.S. Pat. No. 4,253,709 has a large rigidity and can sustain a heavy load at a high speed with a high accuracy. Due to those excellent properties, it has been widely used especially in a field of machine tools.
One of the main features of the linear motion bearing is that it can provide a preload in the steel balls since the diameter of the steel ball is slightly larger than that of the raceway formed between the complementary grooves of the bearing body and the rail. Due to the preload, the assembled rigidity of the structure becomes larger which, in turn, enables to attain high accuracy.
On the other hand, one of the shortcomings of the linear motion bearing is in the usage of a retainer which holds or maintains the steel balls from falling off from the bearing body when it is withdrawn out of the rail.
The shape of the retainer can be either a plate structure as shown in FIG. 8 in U.S. Pat. No. 4,253,709 or a piano wire-like structure extending along the grooves as shown in U.S. Pat. No. 4,929,095.
These retainers are attached either to the bearing body or to the end caps. Because of this, the structure of the linear motion bearing becomes more complicated and more difficult to assemble a saddle from bearing body with steel balls by an automatic process.
Further, if the retainer dislocates its position, it may contact the steel balls and increases the friction. The retainer will be damaged or destroyed eventually. It also increases the cost of the linear motion bearing.
Another problem of the retainer lies in that the size of the radius of the curvature of the groove is inevitably limited due to existence of the retainer in the narrow raceway and so is the load rating.
It is well known that the retainer loses its function and becomes useless once the saddle is fit onto the rail since the steel balls are held solely by the complementary grooves.
Nevertheless, there must be a retainer since if the retainer is eliminated from the linear motion bearing, there is no way to maintain the steel balls within the bearing body of the saddle when it is withdrawn from the rail.
Yet another problem is how to fit the saddle onto the rail while maintaining the loose steel balls.